The present invention relates to high velocity automatic cannon and weapon munitions having a pressure relief system.
1.0 Introduction:
The term “Insensitive Munitions” refers to a generic body of munitions knowledge that includes guidance practices, regulations, technology, methodologies and standards for complying with the following objective:                “To ensure, to the extent practicable, that munitions under development or procurement are safe throughout development and fielding when subject to unplanned stimuli. IM are those munitions that reliably fulfill their performance, readiness, and operational requirements on demand, and that minimize the probability of inadvertent initiation and the severity of subsequent collateral damage to weapon platforms, logistic systems, and personnel when subjected to selected accidental and combat threats.”        
Insensitive Munitions (“IM”) technology includes new energetic materials with less sensitivity to unplanned stimuli as well as mechanical and functional designs that mitigate the undesired reactions against such unplanned stimuli. Two key IM tests required by the US Department of Defense in qualification of ammunition are slow cook-off and hot cook-off tests where the ammunition is exposed to fires and the results are documented.
New munition designs should meet the standards for reducing the risks of unplanned stimuli creating a catastrophic event. There have been various developments of so-called “pressure relief” systems or “venting” devices for contained explosive and rocket motors.
Pressure relief systems in munitions, subject to unplanned stimuli such as elevated ambient temperatures, must act before the unplanned stimuli initiates an unacceptable hazard. For ammunition, the unacceptable hazard is initiation of a primer that ignites the propellant leading to separation and flight of a projectile. In a worst case scenario, the flying projectile arms and detonates. The IM venting systems work by venting the propellant, thereby reducing the efficiency of propellant combustion (burn) and precluding the flight of projectiles. Any such pressure relief system must not interfere with the normal functioning of that cartridge (munition) when fired from a family of automatic weapons.
Different solutions for venting devices or pressure relief systems for IM applications have been designed, tested and applied to a variety of munitions, be they ordnance, rockets or missiles. These solutions include concepts of melting plugs alone or in combination with burst disks, memory alloy fasteners or rings, as well as detonating cords and active venting devices using shaped charges, etc. Many applications of the concepts described have been developed on munitions ranging from grenade cartridge cases, rocket motor cases and artillery projectile bodies, to fuses and warheads. However, few such concepts have been applied to gun-fired munitions and even fewer relate to technology that disables ammunition propulsion systems in response to IM stimuli.
1.1 Prior Art:
The prior art in this field of Insensitive Munitions includes a number of articles and patents that are relevant to the present invention. Typical of such prior art is the U.S. Pat. No. 5,936,189 to Lubbers and an article “IM Solutions for Projectiles Crimped to Cartridges for Artillery Application—Phase II, Transition from Cartridge Case Venting to Insensitive Propeliant” by Carl J. Campagnuolo, Christine M. Michienza, Edward G. Tersine, Christine D. Knott, William J. Andrews—NDIA IM/EM Symposium, May 11-14, 2009.
The Lubbers U.S. Pat. No. 5,936,189 discloses a cartridge munition used with rapid-fire weapons of medium caliber (about 40 mm). Many such cartridges are received into a belt that is fed to the rapid-fire weapon. The propulsion chamber in some cartridge case types are divided into a high-pressure chamber (into which the propulsive charge is placed) and a low-pressure chamber that is connected with the high-pressure chamber via exhaust apertures. The cartridge case and projectile are mechanically connected via a central threaded connection that includes an intended break point. Other two chamber designs (such as the US M430 propulsion) use the age-old technique of crimping a cartridge to a projectile.
When the propulsive charge is ignited in the high-pressure chamber by means of a primer (igniter), the propulsive charge burns and propulsive gases are created at high pressure that then act on the projectile base in both chambers. This drives the projectile out of the cartridge case, after the break point between cartridge case and projectile is broken. A similar cartridge munition is described in Lubbers, U.S. Pat. No. 4,892,038.
The U.S. Pat. No. 7,107,909 and U.S. Patent Publication No. 2008/0006170 A1, both to Haeselich, disclose another concept for venting pressure from the cartridge case of a cartridge munition that renders the cartridge's propulsion inoperable.
Notwithstanding these references, however, the pressure relief concepts disclosed in the prior art generally concern devices for releasing pressure from warheads and rockets.
1.2 Current Concepts and their Limitations:
Most prior art references describe venting concepts for rockets, missiles, mortar rounds and grenade projectiles. None of the disclosed solutions provides both (1) venting projectile cartridge cases in a way that serves as (2) a sound solution that is usable across a spectrum of automatic cannons and weapons. Automatic weapons and cannons generally fire high velocity cartridges such as 12.7 mm SLAP, medium caliber APDFS projectiles where significant heat and pressures occur. The required containment of pressure in a cartridge case varies from weapon to weapon. For automatic weapons, heat is induced (transferred) into the cartridge case as the ammunition progresses through ammunition handling, which includes storage, feeding, chambering, function fire, ejection and extraction. In this case, the cartridge case must survive intact throughout the entire operational cycle. Large caliber projectiles (artillery and tank) fire from fully contained breach mechanisms.
1.2.1 Venting of Cartridge Cases:
Venting devices (IM plugs) with metallic melting plugs in the base of the cartridge, such as that described by Haeaelich in U.S. Pat. No. 7,107,909, are well suited for low internal pressure cartridges fired from a single shot 40 mm low velocity weapon like a M203 launcher. In this sort of hand feed weapon, the ammunition (1) does not undergo stressful ammunition handling (feeding, chambering, extraction or ejection), (2) is not exposed to high breach chamber temperatures, and (3) is not extracted inside of an automatic weapon. The solution described by Haeselich provides adequate strength as the pressures are low and the breach provides good containment and physical support of the cartridge case. The Haeselich design utilizes one or more naked metallic melting plugs made of an alloy combination of bismuth, tin (or lead). Manufacturing controls of the metallurgy provide for a consistent low temperature melting point (around 140° C.). As the metal alloy approaches its melting point, the melting plugs lose their structural strength and cannot withstand the internal pressure of the high velocity projectile in the normal operation mode of the round (function fire from an automatic weapon chamber). In addition to or instead of the use of bismuth, tin (or lead), the melting plugs may use polymers. The use of either bismuth, tin (or lead) alloys may be substituted with certain polymer plugs.
Nevertheless, in most automatic weapons and cannons a naked melting plug (as a method for creating a vent) does not provide:                (1) adequate structural integrity to the cartridge case. Structural integrity is particularly important as some cartridges are exposed to heat during ammunition handling (storage, feeding, chambering, function fire, extraction and ejection). During an automatic cannon's ammunition handling process, heat will soften fusible IM plugs and additional structural integrity is important in most automatic weapon/cannon applications;        (2) solutions for weapons where heat induced by function firing a cartridge will cause the plug in a cartridge case to disintegrate and foul the feeding of weapons;        (3) for precluding the escape of gases through the melting plug in the breach (or bolt). By preventing the breach melting condition, damage to the bolt face (or breach block) is prevented; and/or        (4) optimizing the physical separation between the primer (igniter) and the propellant.        
In many cases automatic weapon and cannon ammunition handling include dwell times that require cartridges to undergo an exposure to heat and even undergo chambering in a hot barrel. Therefore, an effective IM vent must function where automatic fire has heated the bolt and chamber to near the temperature in which soft metal (bismuth, tin or lead) or a specific plastic polymer undergoes a phase change to a liquid. When a phase change metal or polymer is used in non-fusible bursting plugs, the cartridge case can retain adequate structural integrity (support) as the outer walls of the cartridge case are supported by the weapon chamber. The rear of the cartridge case is typically supported by a bolt that chambers the cartridge into a chamber (or breach). Seals and the geometric configuration can provide integrity to the cartridge walls while the melted metal or polymer is in compression. This configuration, an example of which is illustrated in FIG. 16, allows the liquefied metal or polymer, encapsulated by a non-fusible material, to provide structural integrity as the IM bursting plug, while liquid, is in compression during function fire. Conversely, when a cartridge with the IM vent described herein is heated in an unsupported situation (not in a breach or held by a bolt), the IM vents will burst as intended as the liquefied metal or polymer will not be compressed against the metal surface of a weapon and the unsupported bursting plug lacks the structural integrity to contain the propellant burn.
When using memory metals, a parallel design challenge occurs. The heated cartridge and IM vent using memory metal (where it is held in compression by the automatic cannon's chamber and bolt) must provide adequate structural integrity to provide for function fire.
In addition to functioning in the chamber of a hot weapon, the cartridge case and IM plug must allow the ammunition to function properly through the entire automatic weapon cycle (storage, feeding, chambering, function fire, extraction and ejection). It is important that, after extraction, the IM vent does not disintegrate in the automatic cannon or weapon.
It is also beneficial to configure memory metal rings or bursting plugs that house an igniter (primer). In configuring a “support component” to house the primer, a designer can configure the primer to optimize physical separation prior to or preventing containment required to effectively ignite propellant powders.
1.2.2 Automatic Weapon Types, Desire for Common Ammunitions Fired from Different Automatic Weapons, Peak Operating Pressure, Integrity of Cartridges, Variations in Weapon Breaches and Cannon/Gun Chambers:
There are significant differences in the design integrity of weapons chambers and breaches. Additionally, ammunition handling system vary from weapon type to weapon type. Medium Caliber ammunition fires from:                Blow back weapons        Open Bolt weapons        Gatling Guns        Browning Gun Mechanisms        Run Out Gun Mechanisms        Chain Gun (Cannons)        Gas Feed Cannons        
Some weapons completely lock the ammunition into sealed breaches, while other weapons may rely on the integrity of the cartridge case to partially contain the propellant gases. Chain guns and Gatling guns can be both (self powered gas/recoil) and electrically operated. The internal pressure that higher velocity chambers and cartridges must accommodate during normal operation is in the order of 420 Mega Pascal or higher. FIG. 1 shows the burst pressure inside the cartridge case of a 30 mm munition as a function of time. Generally, the higher the internal pressure, the more likely the ammunition will fire from a sealed broach. A bolt frequently rams the cartridge into a chamber or breach providing some structural support to the base of the cartridge case. Under these circumstances, the ammunition may have some dwell in the hot chamber for an automatic weapon.
The design relationship (design constraints) among the breach, chamber and cartridge case varies from weapon to weapon. However, most automatic weapons have the following steps of in ammunition handling (SFCFFEE):                Storage        Feeding        Chambering        Function Fire        Extraction        Ejection        
The following Table describes steps A-G generally used in automatic cannon feeding systems (operations). The design criteria for steps A-D entail the cartridge case providing adequate strength and integrity to provide good sealing and function in the cannon's chamber. Once fired, the design requirement shifts in that the “heated” IM plug must retain adequate structural integrity to preclude disintegration of the IM plug (spilling the melted contents into the weapon). In the case where a memory alloy (or a mix of melting plug and memory alloy) is provided, the IM plug must not disintegrate.
AutomaticWeaponFunction FireHeat Condition During AmmunitionStepTimeStepsHandling and OperationAT0-T1Exposure of Ammunition to If outside a vehicle, ammunition box is heated by the sun.heat in an ammunition box If inside a vechile heat from vehicle operating components(Note 1)frequently transfer heat to cartridges in an ammunition box. BT1-T2Feeding of Ammunition As ammunition nears the heated chamber of a weapon, componentsinto a breachin the ammunition handling sysytem transfer heat into a cartridge.CT2-T3Ammunition dwell in a Closed bolt/breach designs chamber a cartridge where that cartridge chamber or bolt facecan be in a ready position waiting for function fire. In this condition,(see note breach construction)heat (from prior cartridges fired) will heat a cartridge. With open bolt designs a cartridge case is attached to a bolt face. For recoil operated weapons, ammunition, bolt and breach move in a synchronized fashion.DT3-T4Function FireThe ignition of propellant transfers significant heat into the cartridge case.ET4-T5ExtractionAmmunition handling systems extract the spent cartridge from the cartridge case. At this point, the cartridge case is a heat sink carrying a hot spent cartridge from a weapon. The cartridgeis under high g forces as it is removed from the weapon and mechanical components extract the cartridge case via various well known ammunition handling methadologiesFT5-T6EjectionAmmunition is ejected from the weapon. It is generally desirable that the ammunition does not disintegrate in a manner that would foul the weapon or create a safety concern.GT6-T7Collection of Spent Cartridge The cartridge case cools as it moves through the air and lodges Casesagainst a cooler ambient surface.
Note 1: During step T2-T4 various breach designs heavily influence the required structural strength of a cartridge case.
Note 2: Increasing heat is transferred to the projectile and cartridge case as the ammunition undergoes ammunition handling (Steps A-C) in an automatic weapon. Function fire (Step D) imparts a significant amount of heat into the cartridge case. The cartridge case's structural strength required for Steps A-D depends on the design of a breach construction. The structural integrity after firing (Steps E and F) must preclude disintegration of components in an automatic weapon that may affect weapon function. Additionally, depending on the location where spent cartridge cases are collected, it may be desirable that debris is minimized so users may wish that spent cartridge cases do not disintegrate even after ejection.
Note 3: In Stops A-C the cartridge case should retain adequate structural integrity until function fire where the projectile separates from the cartridge case venting gases and propelling the projectile.
Note 4: In Step D the cartridge case should retain adequate structural integrity so that IM plugs (supported by the chamber or breach walls or bolt face) do not fail. The IM plugs should not fail in compression.
Note 5: In Steps E and F the cartridge case no longer must retain the strength of structural integrity required up to function fire; however, the cartridge should still retain adequate structural integrity so that the plug does not disintegrate as undergoes the ammunition handling steps of extraction and ejection. Further, it is very important that melted plug material does not adhere to weapon components where it could foul the weapon or create stoppages.
Note 6: In Step G it is generally desirable that spent cartridge cases retain their integrity so that are easily collected for disposal. The disintegration of materials could create hazardous edges and surfaces.
The variation in weapon designs and need for automatic cannon and weapon ammunition capable of being fired from a broad compatibility in multiple weapon types. This generally requires that a cartridge case retains varying degrees of structural integrity as it undergoes ammunition handling. For NATO countries, automatic weapon and cannon caliber ammunition is generally identified as ammunition in the following diameters: 20 mm, 25 mm, 30 mm. Some products like the 12.7 mm (.50 cal) and 40 mm AGLs are cross-over weapons that can be described as heave machine guns. In some cases, different cartridge case lengths are applicable to different calibers. The following paragraphs provide a summary of the principle cannon weapons in US/NATO:
1.2.2.1 .50 cal (12.7 mm):
The famous .50 cal family of weapons is one of the oldest designs still in widespread use worldwide. Two weapons dominate the market.
Weapons/Cannons Firing 12.7 mm × 99 AmmunitionWeaponRate of FireNomenclatureWeapon Type(Rounds Per minute)M2 BrowningRecoil Operated450-635GAU 21 (M3M)Recoil Operated6000 rpm
1.2.2.2 20 mm Cannons:
Two types of cartridges dominate the 20 mm cannon market; namely, 20 mm×102 and 20 mm×139.
WeaponRate of FireNomenclatureWeapon Type(Rounds Per minute)Weapons Cannons Firing 20 mm × 102 AmmunitionM197 GatlingGatling Gun730 rpmM61 VulcanGatling Gun6000 rpmM621 GiatBlow Back800 rpmWeapons/Cannons Firing 20 mm × 139 AmmunitionKAD OerlikonGas or Blowback600-850 rpm(formerly the Hispano-Suiza HS. 820)M683 GIATDelayed Blowback720 rpmMK 20 Rh 202Gas Operated880-1,000 rpmRheinmetall
1.2.2.2 25 mm Cannons:
25 mm like most cannon calibers must fire from many different types of Weapons/cannons with very different heat profiles, dwell times and ammunition handling systems.
Weapons/Cannons Firing 25 mm × 137 AmmunitionWeaponRate of FireNomenclatureWeapon Type(Rounds Per minute)M242 BushmasterChain Gun200-500 rpmGAU-12 EqualizerGatling Gun1800-4200 rpmOerlokon KBA B02B1Gas-operated 200-600 rpm(Rheinmetall)weaponGIAT 25M811Externally Powered125-400 rpmCam Arrangement
1.2.2.3 30 mm Cannons:
30 mm weapons provide a useful example of the desire for standardized ammunition (within NATO) that guides ammunition design. There are two types of 30 mm cannon cartridges in US DoD service 30 mm×173 and 30 mm×113.
Weapons/Cannons Firing 30 mm × 173 AmmunitionRate of FireWeapon NomenclatureWeapon Type(Rounds Per minute)GAU 8 AvengerGatling Gun4200 rpmBushmaster Chain Gun Chain Gun200 rpmRheinm 30-1/2Gas Operated Weapon700 rpm
In many gas operated weapons (like the Rheinmetall 30-1/2) proper venting of gases is paramount to operation of the weapon.
Weapons/Cannons Firing 30 mm × 113 AmmunitionWeaponRate of FireNomenclatureWeapon Type(Rounds Per minute)ADEN MK4Recoil (electric primer)1200-1700 rpmM230Chain Gun625 rpm
1.2.2.4 40 mm AGLs:
40 mm Automatic Grenade Launchers (AGLs) like the MK19 and MK47 are cross-over weapons. 40 mm AGLs do not fire with the energy of cannons, but the weapons do fire ammunition at a rate of fire of 250-375 rounds per minute. The MK19 that uses an open bolt with advanced primer ignition only the part of the un-chambered cartridge provides structural integrity. An MK47, firing the same cartridge, is a short recoil operating system firing from a closed bolt. Therefore, the MK19's cartridge case requires greater structural integrity for firing than the MK47 as the cartridge case is not fully chambered at the time of primer ignition. An MK47, on the other hand, fires from a closed bolt (at a slower rate of fire) so ammunition fired from the MK47 has a longer dwell time in a heated chamber (breach). It is also important to realize that some weapons (like the MK19) do not automatically eject the last spent cartridge case on a belt (the last cartridge remains on a hot bolt face).
Weapons Firing 40 mm × 53 Ammunition (High Velocity 40 mm)MK19Blow Back/Open Bolt325-375 rpm(advanced primer ignition)MK47Closed Bolt250-300 rpmH&KBlow Back/Open Bolt350 rpm(advanced primer ignition)
1.2.2.5 Ammunition Standardization for Automatic Cannons and Weapons:
One should note, as illustrated in the tables above (.50 cal, 20 mm, 25 mm, 30 mm and 40 mm AGLs), that the weapon rates of fire vary greatly within each ammunition caliber family. NATO standardization is discussed below in paragraph 1.3. With the large variation in rates of fire among automatic weapon families, one will recognize that the heat produced in higher rate weapons is much greater than the heat produced in weapons with lower firing rates. In an environment where standardized ammunition is required to function from multiple weapons, an effective IM vent for medium caliber ammunition must provide for (1) venting functions in slow and hot cook-off, and (2) while functioning across a spectrum of weapons with different action times, different dwell times, where cartridges undergo different g loads as the cartridge case undergoes the storage, feeding, chambering, function fire, extraction and ejection. When considering IM ammunition solutions for ammunition fired from automatic weapons and cannons, the Haeselich design, as disclosed in the aforementioned U.S. Pat. No. 7,107,909, is inadequate. The design is not robust enough in providing structural integrity to function from automatic weapons and cannons. For automatic cannon/weapon ammunition, the design requirements are further explained herein.
1.2.3 Heat Transfers, Chamber Dwell Time and Ammunition Handling:
Some systems, such as turrets in fighting vehicles, often have ammunition feed systems where the ambient “ready” ammunition is exposed to high temperatures. Many closed (unsealed) bolt weapon designs rapidly transfer heat into cartridge cases. Weapons such as the .50 cal Browning and certain artillery types have cook-off dangers where hot barrels rapidly transfer heat to their cartridge cases. Some weapons also have slow rates of fire with extensive dwell times in a chamber. One should also note that the surface (contact area) of “hot” ammunition handling surfaces effect the heat transferred into cartridge cases. The heat produced by previous salvos is transferred into automatic weapons. Accordingly, the cartridge case and IM vent design must accommodate heat transfer into the cartridge case during storage, feeding, chambering, function fire, extraction and ejection.
1.2.3.1 IM Function:
The melting temperature of the meltable metallic or polymer plugs must be equivalent to the temperature induced by a heating of a fire (slow cook-off or fast cook-off testing). Alternatively, the use of memory metal alloy alone or in combination with a memory metal alloy should provide for venting from the cartridge case at a temperature that is lower than that of the auto-ignition in the primer (igniter), flash tube or propellant charge. Heat transfer and elapsed time influence function of the IM vents. It is also beneficial (in terms of IM effect) to the extent practicable to use the primer to energetically open the vent, thereby contributing to inadequate containment and inefficient propellant burn. Use of a bursting component with the metallic or polymer plug is critical to providing structural integrity through the ammunition handling process used in automatic weapons.
1.2.3.2 Projectile Separation:
Another condition is that the venting device must be designed to vent gas at an internal pressure lower than the pressure which drives projectile separation and flight of the projectile when the cartridge is not chambered (or the cartridge is stored in containers).
1.2.3.3 Heat Transfer and Dwell Time:
Weapons differ in the amount of heat induced into the cartridge case during feeding (ammunition handling). Heat flows into the cartridge as it undergoes storage, feeding, chambering, extraction and ejection (SFCFFEE) during automatic function fire. The dwell time in a hot chamber and area of contact surfaces can affect the structural integrity of the cartridge case (with IM plugs). An understanding of heat flow is especially important in automatic weapons cartridge.
1.2.3.4 Target IM Transition Point (Concurrent or After Function Fire):
As heat is transferred at each step of the feeding cycle, the cartridge case (with IM plug) nears the point where structural integrity will be lost. The design goal is to insure that structural containment is not lost prior to function fire. Failure of an IM plug in a chamber may result in erosion and will certainly foul the weapon's breach. For a metallic melting plug configuration, the prior art does not provide for adequate structural integrity to undergo extraction and ejection (without the raw melting plug material from oozing from the cartridge case fouling the feeding mechanisms). Post-firing induction of heat into a cartridge case may cause the IM plugs to disintegrate (melt) and foul a weapon. Quickly after function fire, the heat transferred passes the phase transition point of the melting plug and the internal contents of the plug liquefies. The liquefaction of the IM plug material results in a loss of structural integrity that is critical in some breach mechanisms. It is possible to utilize an insulating metal (like zirconium) that provides insulation to the IM plug fabricated from either a memory metal, a melting alloy or a combination thereof. Depending on a combination of factors (dwell time, heat transfer, maximum chamber temperature, for example) it may be necessary to conduct heat flows around an IM plug, thereby delaying the time period for activation of the IM plug. FIGS. 13A and 13B illustrate this timing for memory metal and fusible material, respectively.
1.2.3.5 Pressure:
In order to provide a context for the description of the high loads due to the internal pressures of certain types of weapons, and hence to understand the requirements of an IM venting design to withstand these loads, reference is made to FIG. 2, which is a table of values of burst pressures in the cartridge case for a variety of weapons and munitions.
1.2.3.6 Breach, Bolt Face and Function Fire:
The relative pressure, sealing of the breach, mechanical support provided by breach and bolts, dwell times and heating of the cartridge through the temperature cycle all influence the required structural integrity of an IM cartridge case. Where a weapon has a fully sealed breach, the breach wall and bolt face will provide important structural support (containment) of the cartridge case. In some cases, chambering into a hot breach may result in liquefaction of the fusible material in an IM plug; in this event, the bursting plug must provide for adequate structural integrity (in compression) so that the IM plug fill does not fail. Failure would spill melted material and foul the weapon mechanisms and chamber when the “spent” cartridge case undergoes extraction and ejection.
1.2.4 Risk of Residue and Weapon Fouling/Stoppages:
It is important to recognize that a melting plug should not leave residue and should not melt (or otherwise disintegrate) during SFCFFEE. After function fire ammunition undergoes ejection and extraction, the cartridge case may undergo significant g loads. The disintegration of the cartridge during post firing extraction or ejection will foul automatic weapons mechanisms. Therefore, it is desirable to have structural integrity of a melting plug through the entire post firing ammunition handling cycle (extraction and ejection). There are also shortcomings to the cooled “spent cartridges” having hazardous rough edges and surfaces.
1.2.5 Large Caliber Applications:
Some large caliber devices use autoloaders, but many other cannons still rely on human operators to feed, chamber, extract and eject the ammunition. There are experimental solutions for 105 mm Howitzer projectile cases using metallic and polymer melting plugs. See, e.g. NDIA briefing by Carl J. Camagnuolo May, 2009, posted at:    www.dtic.mil/ndia/2009insensitive/5Bcampagnuolo.pdf
It is possible that fully contained breaches that utilize Haeselich vent plugs from polymers might use melting plugs that fully vaporize during ignition; however, it is obvious that the naked bismiuth tin (or polymer) IM plugs will melt immediately after ignition and the resulting residue will foul chambers, breaches, weapons and complicate material handling. Current polymers have carbonized under the high flame temperature of burning 105 mm propellants.
1.2.6 Context of Design Challenge in Automatic Weapons and New Disclosed Art:
The fundamental design challenge to incorporate IM venting into medium caliber ammunition is identification or novel arrangements that provides:
(1) Optimized venting of the cartridge case when exposed to outside stimuli. It is desired to maximize the venting area and use the energy of the primer/igniter to enhance venting;
(2) Sound structural integrity of the cartridge case up to the point of ignition (in a automatic weapon chamber); and
(3) Retention of adequate structural integrity (after the cartridge case is heated by function firing) to preclude disintegration in chamber as the cartridge case will undergo g forces when extracted from the chamber and ejection from the weapon. In this case, the structural integrity must preclude fouling of the weapon).
1.2.7 Limited Application of Haeselich:
In weapons with certain characteristics, the Haeselich design does not provide adequate structural integrity required to preclude catastrophic failure, venting propellant gases. The following three factors strongly influence an IM vent's design parameters for an ammunition type's cartridge case:
(1) Integrity of Chamber:
Some weapon/cannon chambers (breaches) are “sealed” while other weapons feed and ignite the cartridge case prior to the cartridge being fully chambered. This is sometimes described as a “closed bolt” versus “open bolt” design when discussed in the context of machine gun design. Further, design of breaches provides varying integrity. Automatic cannons have varying arrangements and the integrity of chambering and sealing varies across calibers.
(2) Pressure and Structural Integrity in Feeding and Chambering:
A cartridge's propellant gases generate high pressure. An MK19 HV cartridge will generate about 90 Mpa of pressure whereas the pressure of both medium-caliber artillery and tank ammunition varies between 350 Mpa to 650 Mpa.
(3) Induced Heat:
The heat energy transferred into the cartridge from the weapon during storage, feeding, chambering and firing requires an improved strength of design (integrity) of a cartridge. As the ammunition handling system moves the cartridge through different stations leading to chambering, the cartridge case has physical contact with ammunition handling systems and the automatic weapon (cannon) chamber. The dwell time and contact surface area of the ammunition during feeding and chambering affects the transfer of heat. Longer dwell times increase the transfer of heat into a cartridge case. During function fire a significant amount of heat is transferred into the cartridge case.
As the cartridge case ejection accelerates a “spent” cartridge case from the breach and from the weapon, such ejection carries heat away from the weapon. During post firing ejection, it is desirable to preclude melting plugs from disintegrating in the weapon, thereby leaving residue that will foul a weapon.
(4) Low and Medium Heat Systems:
An example of a low heat, low pressure, minimum dwell time projectile is the 40 mm M203. The MK19 MOD 3 system, prevalent with 40 mm weapons, is an open bolt design, where the cartridge fires in an advance primer ignition system. Therefore, 40 mm HV ammunition fired from a MK19 MOD 3 is never chambered into a hot breach. However, the cartridge may remain in the ready position on a MK19 bolt, thereby transferring some heat from the bolt to the cartridge case.
(5) High Heat/High Pressure Automatic Weapons:
Two examples of cartridges generally exposed to a high heat, high pressure system are (1) a .50 cal (12.7 mm) Browning weapon, (2) a 25 mm×137 cartridges fired from the 25 mm Bushmaster series weapons and GAU 12 weapons, and (3) 30 mm×173 weapons fired from 30 mm Bushmaster weapons, Rheinmetall weapons and GAU 12 weapons. In this case two different types of weapons require the ammunition to function (and the cartridge case to retain integrity) while the ammunition is exposed to increasing heat and under high pressures. In these two examples, it is important that the cartridge case maintain adequate structural integrity through the entire cycle of weapon feeding, chambering, function, extraction and ejection. When the cartridge functions, significant heat is transferred into the cartridge case. The expelled cartridge case carries heat from the weapon.
Any attempt to incorporate the Haeselich IM solution into most medium caliber weapon/ammunition combinations will not work as the solution does not provide adequate structural integrity through the entire SFCFFEE cycle. Therefore, the potential application of the Haeselich design with automatic weapons is very limited.
1.3 Other Shortcomings of Haeselich:
The Haeselich U.S. Pat. No. 7,107,909 does disclose an IM design with sufficient structural strength to enable the cartridges to function adequately at low pressures in low heat single shot weapons. However, the design does not provide adequate structural integrity for broad application in automatic cannons and weapons. In addition to the shortcomings identified in paragraphs 1.2, the Haeselich melting plug approach has other practical shortcomings and limitations:
(1) The venting areas are small, requiring the provision of multiple plugs in a cartridge case; and
(2) The venting device does not provide for a physical separation of the primer from the propellant powder.
(3) The actual process of igniting a cartridge rapidly heats a cartridge case. In firing, a tremendous amount of heat is transferred into the now “spent” cartridge case. When the “heated” cartridge is extracted and ejected, heat is carried away from the chamber of the automatic cannon. It is desirable that the “spent” cartridge case have adequate structural integrity so that the naked melting plug does not disintegrate, allowing ejection of the cartridge case in a manner that keeps the weapon free of debris and materials. Splatter from melted alloys or carbonized polymers can foul weapons.
Generally speaking, it is possible to identify primers (igniters) that under heating will initiate before powder burning. In this case, it is beneficial to use the action of the primer initiation to physically propel the plug primer sub-assembly away from the cartridge creating a greater physical separation from the powder.
For the previously disclosed designs, it is important to understand that heat “builds up” during the firing process. Cartridges are generally chambered into a weapon that may have a great deal of heat. Heat is quickly transferred through thin-walled cartridge cases. In this case the Haeselich solution is optimized for 40 mm LV ammunition fired from M203 type launchers.
While the Haeselich design has been an important step forward and functions with single shot, low pressure, low heat producing projectiles like a 40 mm×46 LV cartridge fired from an M79, M203, M320 single shot launchers or similar weapon, it is desired to have IM venting solutions that allow for a broad use of cartridges in automatic cannons and automatic weapons. Robust solutions will provide for IM venting in an environment where cannon caliber ammunition must undergo stressful ammunition handling and function from many different types of automatic cannons and weapons. Users in the United States Department of Defense and NATO militaries have standardization programs and promulgate STANAGS (Nation Standardization Documents) that provide requirements for ammunition computability among NATO militaries. Generally, the NATO standardization documents (STANAGs) set requirements by caliber and ammunition type for function fire compatibility among multiple automatic weapons.
NATO STANAG Ammunition Compatibility Documents by CaliberAmmunition TypeNATO Document.50 cal 12.7 mm AmmunitionSTANAG 438325 mm × 137 AmmunitionSTANAG 417330 mm × 173 AmmunitionSTANAG 462440 mm × 53 HV AmmunitionSTANAG 4403
1.4 Design Objectives of the Present Invention:
For the cartridge designer working to optimize IM venting (in slow cook-off and fast cook-off conditions), the fusible material must liquefy for the IM vent to become “operational.” When discussing ammunition propulsions undergoing slow cook-off conditions the propellant generally becomes unstable and initiates the 1st energetic event. In fast cook-off conditions, the primer (or igniter) may initiate first energetic event. Generally speaking, the IM event will occur at lower temperatures in slow cook-off testing. Liquefaction of the IM fusible material at a temperature in the range of 140° C. results in a reduced structural integrity in the IM vent with busting plug. With the 1st energetic reaction (either primer/igniter initiation or propellant burn) the bursting plug fails, venting the expanding propellant gases. One can generally expect slow cook-off initiation to take place after a cartridge reaches 140° C. Better propellants could eventually increase the temperature where slowly heated propellants ignite; however, the temperature of 140° C. is identified herein as the temperature range that IM cartridge case should vent.
Cartridge function (key parameters):                Integrity of strength up to function fire;        Integrity of strength (post function fire) to preclude disintegration during extraction and ejection;        Maximum temperature of the cartridge through SFCFFEE; and        Cost effectives of the solution.        
Again, it is critical that the design retains adequate cartridge case structural integrity through the cycle of feeding, chambering, function fire, extraction and ejection (automatic weapon fire). The need for structural integrity extends to post function fire extraction and ejection to preclude disintegration of materials after function fire that could lead to fouling of the weapon (or other stoppages).
To summarize, an efficient solution for a venting device for munitions, including high velocity projectile cartridges with high internal pressures (with higher heat conditions found in automatic weapons), must achieve the following operating conditions in order to provide an IM Class V response:
(1) The venting device in the base of the cartridge case should provide the same structural integrity as a standard case when it is feed, chambered, fired, extracted and ejected from an automatic weapon.
(2) The venting device should perform its function to expel the gases at a cook off temperature lower than the one which produces the auto-ignition of the secondary explosive of the booster and hence the main propellant charge.
(3) The venting device should perform its function, creating a large venting area such that the internal pressure in the cartridge produced by an accidental ignition of any of the propellant (energetic) materials in the cartridge never exceeds the value of the internal pressure in the cartridge that would cause the projectile to separate and be propelled through the air with substantial velocity.